Next generation imaging systems are required to have Charged Coupled Devices (CCD's) that operate at very high frequencies and to have a high resistance to radiation flux (the imager is said to be radiation hardened). Many of these applications are in space where radiation flux is significant and reliability is critical. State-of-the-art imagers are constructed as Si integrated circuits in the form of CCD's or active pixel arrays. In the CCD, the pixels are clocked sequentially to a common output amplifier. In the active pixel array, the array is x-y addressable and each pixel is output to its own dedicated amplifier (the array is output on a row by row or column by column basis).
Si technology is limited by the presence of the silicon oxide in both the active and passive regions of the integrated circuit in a number of ways. A main limitation is the sensitivity of the oxide to radiation flux. The radiation creates traps and other charged defects in the insulator which alter the internal voltage thresholds in both active and passive regions within the integrated circuit. After a certain cumulative exposure level, these threshold changes render the circuit inoperable. The gate oxide creates limitations in other ways as well. The Si CCD couples one pixel to the other via overlapping gates. Each overlapping gate creates a small region of thicker oxide between pixels which inhibits charge transfer and therefore sets a speed limitation upon the CCD. These oxide barriers are fundamental to the Si CCD and constitute a transfer speed limitation. Some approaches have been employed to eliminate these effects such as the virtual phase CCD. However, these structures are then faced with barriers created by implant misalignment and a lack of well capacity. In any event the transfer speed in the Si CCD rarely exceeds a few MHz.
A further limitation of the Si CCD is its spectral sensitivity. The Si CCD absorbs radiation across its energy gap and therefore is insensitive to radiation with a wavelength longer than about 1 um. It is also insensitive to UV radiation.
III-V device structures based upon GaAs substrates have the potential to overcome the above limitations. In particular, the GaAs CCD has the potential to absorb within a quantum well between the various subbands. This provides the GaAs device with unique capabilities of intersubband absorption and sensitivity in the mid wavelength infrared, long wavelength infrared and very long wavelength infrared regions. The GaAs device structures that currently perform the intersubband detector functions are the QWIP (quantum well infrared photodetector) devices. Two significant limitations of the QWIP as currently implemented are the existence of a significant level of dark current that necessitates cooling of the device to 77K and the fact that the device is not compatible with GaAs integrated circuits. When originally demonstrated the QWIP was considered advantageous because of its potential compatibility with GaAs integrated circuits. However, this compatibility has never been established and so present technology combines the GaAs QWIP wafer in a hybrid fashion with a Si read-out integrated circuit. There have been several efforts to build CCD shift registers using the basic transistor structures of the MESFET and HEMT devices. These technologies have always been plagued by the problem of low transfer efficiencies between pixels in the array. The proposed solutions have been to utilize a resistive coupling between pixels, which would provide drift aided transfer. The problem has been that no viable technique to implement resistive coupling has been found. The use of deposited resistive layers was attempted but the resistive control problems discouraged further investigations.
It is an object of this invention to provide a CCD technology in a III-V semiconductor system which is capable of very high transfer rates, substantially improved maximum charge compared to Si technology, and substantially improved radiation hardness properties compared to Si technology. This CCD technology will implement the resistive coupling structure in a natural way via the crystal growth which optimizes the charge transfer efficiency.
It is another object of this invention to provide a CCD technology which may absorb radiation in the broad range from 3 um-20 um via the intersubband absorption mechanism and convert the radiation to packets of charge for transfer to the CCD output amplifiers
It is another object of this invention to provide a CCD technology which has optical sensitivity in the uv, visible and near IR portions of the electromagnetic spectrum
It is another object of this invention to implement the CCD in an integrated fashion together with HFET technology such that analog to digital conversions can be made on the data.
It is another object of this invention to implement the CCD as part of a monolithic optoelectronic integrated circuit such that a vertical cavity surface emitting laser is available at the output amplifier of the CCD to facilitate the coupling of digital data from the chip into an optical fiber for transmission.
It is a final object of this invention to implement an active pixel within a III-V optoelectronic technology which is sensitive by bandgap absorption in the uv, visible, and near IR portions of the spectrum and sensitive by intersubband absorption in the MWIR and LWIR.